Ischemic Heart Disease

Ischemic heart disease refers to a set
of syndromes intimately related but of diverse aetiology, characterised
generically by an imbalance between supply and demand of oxygen and substrates
in cardiac tissue. This imbalance leads to a production deficit of ATP,
necessary for contraction, and excessive accumulation of metabolic waste
products.

Generally, the obstruction of blood flow
at the level of a coronary artery, caused by the deposition of atheromatous
plaques on the walls of the vessel, appears at the base of this set of diseases
that presents three clinical manifestations (Felker, 2002):

Angina pectoris. The blood supply decreases enough not
to satisfy an eventual demand caused by a situation of stress or stress. It
manifests as transient chest pain due to ischemia, but its effects are
reversible at the tissue level if normal flow is restored.

Sudden cardiac death.Ischemic heart disease appears behind
more than 50% of these events. After disruption of coronary blood flow,
ventricular fibrillation causes the individual to die within the first hour
after the obstruction.

Socio-economic impact of heart disease of ischemic origin.

Currently, in the non-industrialized
regions a number of major infectious diseases cause more than twelve million deaths a year. In the industrialized regions,
these account for less than two million and are equivalent to those derived
from industrialization itself. In this heterogeneous scenario, it is striking
that ischemic heart disease still accounts for 2.5 and 3.5 million annual
deaths in developing and developed areas, respectively. Far from reducing these
figures, future projections predict that increasing longevity and change in
lifestyles will take these numbers increasingly higher.

In 2006, J. Leal et al. published one of the most comprehensive studies to date on
the socioeconomic impact of cardiovascular diseases in the European Union (Leal,
2006). According to official data, cardiovascular diseases accounted for 12% of
total health expenditure in 2003, of which more than a quarter were justified
by heart disease of ischemic origin, a proportion that is maintained and even
increased in other regions. This year, expenses and losses from heart disease
of ischemic origin were estimated at 44,725 million euro for the whole European
Union, and one third of this amount was due to production and productivity
losses due to mortality and morbidity (Leal, 2006).

In the United States, with a population
equivalent to 70% of the European population, heart diseases of ischemic origin
underlie one of every five deaths, and the gross expense in cardiovascular
diseases doubles that of Europe and is twice the internal expense derived from
all types of Cancer (American Heart Association Statistics Committee and Stroke
Statistics Subcommittee, 2010). In emerging countries such as India, China,
Brazil, Mexico and South Africa, cardiovascular diseases are already an
important socio-economic factor and together, 21 million years of future
productive life are lost annually in these five countries (Gaziano, 2007). At
present, sub-Saharan Africa is the only economic region in which this set of
pathologies is not the leading cause of death. If current trends continue, it
is estimated that cardiovascular diseases will be soon a significant factor of
imbalance for many economies (Jaffer, 2003). It is not surprising that, year
after year, a large part of the health budget is dedicated to the prevention,
diagnosis, treatment and research of cardiovascular diseases and, in
particular, heart diseases of ischemic origin.

Historical perspective of ischemic heart disease.

In 1768 William Heberden, with his essay
on angina, published the first detailed study of a heart disease of ischemic
origin. More than 140 years later, in 1912, James B. Herrick published the
first paper describing a case of acute nonfatal myocardial infarction three
years after the introduction of the electrocardiogram (ECG). In this paper,
Herrick described myocardial infarction as a cardiac ischemia with frequent
survival, and accompanied of a damage that was often not irreversible (Herrick,
1912). This study was perhaps the starting point or inspiration of works and
technological advances that, during the 20th century, have contributed to
enrich the knowledge about this set of affections. In the 1930s, it was
introduced the ECG using precordial leads, one of the cornerstones in the early
diagnosis of myocardial infarction. In the 1950s, it was introduced the use of
external defibrillators and pacemakers, which were fundamental in the treatment
of this disease, as well as the first coronary arteriographies. Previously, in
1948, 5,209 people in Framingham (United States) were recruited to be part of
the first prospective study that sought to unravel common patterns in the
development of cardiovascular diseases (Dawber, 1951). On December 3, 1967,
Christiaan Barnard performed the first successful heart transplant among humans
in Cape Town, South Africa. Cardiac transplantation was a tremendous advance in
the care of patients with severe heart failure but nevertheless, it is an
extremely risky surgery to replace an organ that has lost its functionality
irreversibly, and therefore it is a solution for cases with no alternative. In
the early 1970s, techniques that are still in use, such as reperfusion via catheterism
and coronary bypass, were introduced into clinical practice aimed to restore
blood flow in the region affected by the infarction. At the end of the same
decade the use of the creatine phosphokinase test was established for diagnosis
and β-blockers (β-adrenergic antagonists) began to be prescribed to patients
who had undergone angina pectoris or myocardial infarction. Its use would be
generalized during the following decade, alongside the introduction of coronary
angioplasty and the use of thrombolytic agents. The 1990s would see the generalization
of inhibitory agents of the renin-angiotensin system, the use of coronary
stents and the implementation of blood such as the troponin T test.

Classic therapeutic strategies to
myocardial infarction were characterized for their focus towards the arrest of
the processes triggered after the event of ischemia, but none of them offered
the restoration of the pre-ischemic state. These approaches have significantly
reduced mortality and improved the quality of life of patients with myocardial
infarction. Unfortunately, these patients continue to suffer from limitations
that indicate that there is still a long way to go in the treatment of this
disease. Several different strategies have been proposed to replace the heart
or part of the affected heart tissue. The first of these is, as already
mentioned, heart transplantation. Currently, an average of 3,500 heart
transplants are performed every year, but some 800,000 people are still waiting
for an organ transplant (Reiner Körfer [TV interview], 2007). Therefore, although
subject to strong debate, the use of organs of animal origin, or
xenotransplantation, has been proposed as an alternative to transplantation
between humans. This option has been officially implemented in eleven occasions
using hearts of different origins (Deschamps, 2005; Bailey, 1985). Immunological
barriers and the ethical and ideological concerns underlying such practice, and
the still not excluded possibility of xenozoonosis, have made this practice
fall into oblivion from a clinical perspective.

In the late 1990s, with the development
of molecular and cellular biology, cell therapy has been proposed as the first
realistic strategy for restoration and regeneration of cardiac tissue (Melo,
2004). This practice postulates the use of cells of different origins that,
after their injection into the cardiac environment, would act as restorative
agents of the contractile function of the myocardium, thus avoiding cardiac
failure. However, after a very optimistic beginning, the heterogeneity of the
results and methods have made this practice questioned by many authors (Abbasi,
2011). In parallel, new tissue engineering technologies have already proposed
the use of bio-artificial hearts, created from the ex vivo culture of cells with cardiomyocyte capacity on natural
decellularized matrices of animal origin (Ott, 2008). With these strategies
still in preclinical stages, their application as a realistic therapy is still
a prospect for the future.

The advent and establishment of high
performance molecular biology techniques since the late 1990s has also been one
of the major advances in the diagnosis and molecular knowledge of the processes
that underlie and follow myocardial infarction. The identification of protein
markers present in the serum is useful for the rapid and efficient diagnosis of
many patients, thus shortening the waiting times and refining the treatments.

Physiopathology of ischemic heart disease

If for any reason one of the coronary
arteries is blocked, the myocardium runs out of blood and therefore becomes
necrotic. The point at which the obstruction occurs will be determinant for the
survival of the individual since the closer the aorta is, the greater the
volume of tissue affected and the worse the prognosis (Lee, 1995; The GUSTO
investigators, 1993).

Category

(*All
patients received reperfusion)

Arterial topography
of the obstruction

Mortality
(%)

30 days

1 year

Proximal LAD

Proximal to first septal

19,6

25,6

Medial LAD

Distal to first septal, proximal to diagonal

9,2

12,4

Distal/diagonal LAD

Distal to diagonal

6,8

8,4

Inferior moderate/severe (posterior, lateral,
rigth ventricle)

Right coronary or circumflex

6,4

8,4

Inferior (small)

Right coronary or circumflex branch

4,5

6,7

Immediately after the interruption of the blood flow, the myocardium of the affected region stops contracting and in many cases compromises cardiac function. In an attempt to maintain organ homeostasis, the region affected by ischemia, but also the regions bordering it, will undergo a series of processes aimed at compensating the loss of contractile work in a process known as cardiac remodelling (Meijs, 2007).

Cardiomyocytes have an extremely limited division
capacity, and following various autocrine and paracrine signals, become hypertrophic
(Kessler-Icekson, 1984). Initially, cardiomyocyte hypertrophy contributes to
the maintenance of cardiac function. However, this additional effort translates
into increased oxidative stress and the entrance in a spiral that affects a
growing area of tissue, eventually leading tissue to apoptosis and total loss
of contractile function. Moreover, cardiomyocytes stay connected by gap junctionsthat allow for a rhythmic
contractile work (Sheikh, 2009). If part of the tissue loses this capacity, the
rest of the heart will find problems to maintain an adequate and coordinated
rhythm, leading in many cases to fibrillation and new episodes of remodelling.
In a final stage, the whole organ will lose its functionality (Rossini, 2010).

Cardiac fibroblasts, on the other hand, differentiate into
myofibroblasts, proliferate and begin to synthesize proteins of the contractile
apparatus and the propagation system of the electric impulse (Weber, 1997;
Vasquez, 2011). The acquisition of myogenic potential allows them to
temporarily contribute to the contraction, but in a damaged tissue environment,
the coupling of these cells is deficient, eventually contributing to the
appearance of severe arrhythmias that further compromise cardiac function (Rossini,
2010). In the area of ​​focal damage, after an acute inflammatory process that
extends over the first 3-4 days after ischemia (Matsui, 2010), the ventricular
wall loses its contractile functionality, but also its robustness. At the risk
of definitively compromising tissue contractibility, the myofibroblasts beging
to synthesize the extracellular matrixthat
will constitute the fibrotic scar, in order to avoid rupture of the tissue and
subsequent total organ failure.

When cardiac tissue is subjected to
ischemia, the processes of arteriogenesis (i.e. remodeling of pre-existing
vessels) and angiogenesis (i.e. proliferation of new capillaries) are
activated. The activation of pro-angiogenic molecular pathways induced by
hypoxia is a protective mechanism that ensures the supply of nutrients in the
affected tissue (Cao, 2010). For example, HIFs (hypoxia inducible factors), are
transcription factors whose expression is activated under conditions of hypoxic
stress and are capable of inducing the expression of genes such as VEGF
(vascular endothelial growth factor), that promote angiogenesis. However, when
the tissue undergoes extreme aggressions, the mechanisms of homeostasis that
are effective for normal tissue maintenance are insufficient. In fact, acute
responses to tissue hypoxia may not necessarily be beneficial for functional
tissue recovery. Thus, the proliferation of new vessels in response to hypoxia
is accompanied by an increase in vasodilator factors such as nitric oxide and
VEGF. These molecules increase vascular permeability and may lead to the
formation of edema, a determinant mortality factor following myocardial
infarction (Eriksson, 2003; Senger, 1986).

Molecular mechanisms of cardiac remodeling.

The renin-angiotensin system (RAS), the
TGFß (transforming growth factor beta) signaling pathway and theß-adrenergic
system are key mediators of the adaptation of the heart to hemodynamic
overload, and are therefore critically involved in the pathogenesis of ischemic
heart disease (Rosenkranz, 2004). These three regulatory pathways, and
particularly the first two, do not act in isolation, but are coordinated within
a common regulatory system that promotes cardiac remodeling. Thus, various experimental animal studies (Nakamura, 2003; Kim,
2001)and clinical trials (McMurray, 2003;
Pfeffer, 2003) have shown that inhibition of angiotensin II (Ang II) by
administration of angiotensin converting enzyme (ACE) inhibitors or antagonists
of the type I angiotensin receptor (AT1) prevent, mitigate or reverse
ventricular remodelling, and increase the survival of patients who have
suffered myocardial infarction.

The effector molecule of RAS is
angiotensin II, which sees increased expression in situations of mechanical
stress such as hypertension or myocardial infarction. However, Ang II
stimulates different responses in cardiomyocytes and cardiac fibroblasts, and several
studies have concluded that this molecule is not the ultimate trigger of the
responses observed in myocardial cells. In contrast, Ang II induces the
expression of various growth factors such as TGFß, which acts locally through
autocrine and paracrine activation of genes leading to cardiomyocyte growth,
proliferation of fibroblasts and their transition to myofibroblasts, and the
induction of the expression of extracellular matrix proteins (Rosenkranz, 2004 ; Wenzel, 2001). Additional
studies have shown that knockout animals for TGFß do not develop cardiac
hypertrophy in the presence of Ang II (Schultz, 2002).

The synthesis of Ang II requires the
presence of a precursor peptide (angiotensin) that is processed by cathepsin or
renin to Ang I. Finally, ACE catalyzes the hydrolysis Ang I Ang II (Sun, 2009).
Following the recognition of Ang II by AT1R (Ang II receptor), the induction of
TGFß expression, depends on the activation of protein kinase C (PKC), in a
process studied in detail by Wenzel et al
(Wenzel, 2001).

Although different types of cardiac
cells can produce TGFβ, macrophages are the major producers during the earliest
stages of repair. After the acute phase, this task is assumed by fibroblasts
(Sun, 2009) and it will be the performance of this molecule on the same cells
that triggers and leads to the process of fibrosis and extracellular matrix
deposition that contribute to cardiac remodeling. TGFβ includes three isoforms
with high homology (TGFβ1, 2 and 3), which are secreted in their inactive form.
The active form of TGFß is a 12kDa molecule synthesized together with the
latency-associated peptide (LAP), from which it will be released by proteolytic
action of plasmin or thrombospondin 1 (Sun, 1998). Latent forms of TGFβ may
also appear to be linked to other latency associated proteins such as LTBPs
(Latent TGF-binding proteins) (Oklü, 2000).

The active form of TGFß binds to a
membrane receptor complex formed by a heterodimer of TGFß receptors receptors
type I and type II (TGFBR1 and 2). TGFBR1 possesses kinase activity and binding
of TGFβ to the complex results in the phosphorylation of SMAD2 and SMAD3, known
as receptor-activated SMADs (R-SMADs). Activated forms of SMAD2/3 bind to SMAD4
and are translocated to the nucleus where, in association with other general
transcription factors (Sun, 2009) act as transcriptional activators of a large
part of the genes involved in the post-ischemic remodelling of the myocardium.
On the other hand, SMAD6 and SMAD7 compete with R-SMADs for binding to TGFBR1.
This attenuates the transcriptional activation induced by TGFβ. Moreover, TGFβ
is able to regulate its own action by transcriptional induction of SMAD7 (Sun,
2009; Yan, 2011).

TGFß activates other non-SMAD-dependent
pathways (Zhang, 2009) and, in addition to the established TGFß-SMAD signalling
pathway, it is known that TGFß and Ang II can cause even the activation of
non-TGFBRs-dependent pathways (Bhattacharyya, 2009). These lead for example to
the induction of the expression of CTGF (connective tissue growth factor), a
growth factor that participates synergistically with TGFß in the fibrosis
process and matrix generation in connective tissues (Mori, 1999).

In addition to TGFß, various chemokines,
cytokines and growth factors play key roles in repairative cardiac fibrosis.
Many of them, such as MCP-1 (monocyte chemoattractant protein 1), are essential
regulators of the fibrotic process in moderate ischemic events, which do not
cause the massive death of cardiomyocytes (Dobaczewski, 2009). In contrast,
endothelial-derived fibroblasts are able to synthesize endothelin-1, which
contributes to the recruitment of new fibroblasts by EndMT (endothelial to
mesenchymal transition), exacerbating fibrosis (Widyantoro, 2010).

Strategies for the attenuation of the fibrotic response.

The ability to repair damaged tissues
without subsequent scar formation would be ideal and is the ultimate
therapeutic goal. However, even in tissues with considerable regenerative
capacity, the repair of massive or chronic damage based solely on regeneration
of parenchymal cells is not feasible. It is for this reason that the
development of therapeutic strategies that minimize the progression of fibrosis
and scar formation without affecting the total repair process would represent a
great advance. Nevertheless, evidence demonstrates that, if fibrosis is
advanced enough, restoring normal tissue architecture is not possible (Wynn,
2008).

One of the usual emergency strategies
used in patients with acute ischemia is the reestablishment of blood flow
through reperfusion of the obstructed artery. However, although it is an
extremely effective procedure, re-entry of tissue into normoxia leads to a
level of oxidative stress that can not be absorbed by cardiomyocytes, which
enter into apoptosis. It has been demonstrated that the reperfusion process is
responsible for part of the tissue damage of the organ and in fact, several
studies have shown that local or remote ischemic preconditioning prior to the
restoration of blood flow results in an attenuation of the reperfusion injury
(Hausenloy, 2011).

One of the reasons that have been
suggested as the cause of the inability to restore fibrotic tissues is that the
advanced stages are frequently hypocellular (Gandhi, 2011). During the acute
inflammation phase, the presence of macrophages ensures a continuous source of
matrix metalloproteinases that maintain the equilibrium between synthesis and degradation
of extracellular matrix (Navalta, 2006). When these inflammatory mediators
disappear, the cardiac fibroblast (myofibroblasts) remain the only source of
MMPs, causing extracellular matrix deposition to prevail on degradation. The
affected tissue enters a chronic phase of irreversible scarring and the
connective tissue causes an increasing isolation of the cells, until they enter
into apoptosis (Müller, 2011). Treatments aimed at attenuating the fibroblast
response to ischemic events may offer a realistic approach to ischemic heart
disease and in fact, various drugs currently in use are capable of disrupting
or modulating part of the fibroblast response. Unfortunately, their unspecificity,
combined with important side effects, still make recommendable the development
of new agents that improve the prognosis of patients suffering from this
ischemic heart disease (Porter, 2009).